Sonic Enhanced Oil Recovery System and Method

ABSTRACT

To increase oil recovery from an oil reservoir, an acoustic transmitter is disposed in a source well and an acoustic receiver is disposed in a producing well. A portion of the oil reservoir is disposed between the source well and the producing well. An acoustic signal is transmitted from the acoustic transmitter at frequencies of 30 Hz and greater. The transmitted acoustic signal is received by the acoustic receiver and a resonant frequency of the portion of the oil reservoir is determined based on attenuation of the transmitted signal. The acoustic signal is transmitted from the acoustic transmitter at the determined resonant frequency to reduce a boundary layer effect between oil in the oil reservoir and a surface of a substrate in the oil reservoir and between the oil and a brine interface in the oil reservoir.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/377,713, filed on Aug. 27, 2010, titled “SONIC ENHANCED OILRECOVERY METHOD,” the disclosure of which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention generally pertains to the recovery of oil from asub-surface oil reservoir.

BACKGROUND OF THE INVENTION

The production of crude oil from a formation is initially supported bythe expansion of fluids in the pore system and then, as the reservoirpressure falls below the bubble point of the oil, the expansion ofsolution gas provides pressure support. This phase of the reservoir lifeis called primary recovery. Some reservoirs are connected to an aquiferand the flow of water from the aquifer provides pressure support todisplace the crude oil to the producing wells.

As the production rate of crude oil declines under primary recoverymechanisms, secondary oil recovery techniques are used to providepressure support for the oil reservoir. The most popular technique iswater injection into the oil zone and is called water flooding. For highviscous oils, steam flooding is used to provide pressure support, reducethe thermal viscosity and increase the mobility of the oil. For lighteroils, gas injection can be used to induce gravity drainage of the oiltoward the structurally lower production wells and this method is callgas assisted gravity drainage; however, if steam is the injected gas, itis called steam assisted gravity drainage.

In order to improve the ability to recover oil above that normallypossible with secondary recovery techniques, tertiary oil recoverytechniques are used. A tertiary method commonly used in zones beingwater flooded includes the use of diversion agents such as polymers toincrease water viscosity and plug off swept zones to improve verticaland horizontal sweep efficiencies. To mobilize residual oil in the areasalready swept by water, surfactants and caustic agents are mixed withthe injected water to reduce surface tension, but absorption of theexpensive surfactants on clay particles limits the application tocleaner formations. This type of flood is called an alkaline, surfactantand polymer flood (ASP flood).

Unfortunately, these prior art procedures are tedious, time consuming,expensive, and/or fail to recover much of the oil present in oilformations. Accordingly, it is desirable to provide a method andapparatus capable of overcoming the disadvantages described herein atleast to some extent.

SUMMARY OF THE INVENTION

The foregoing needs are met, at least to a great extent, by the presentinvention, wherein in one respect an apparatus and method is providedthat in some embodiments improves the recovery of oil from oilformations.

An embodiment of the present invention pertains to a method ofincreasing oil recovery from an oil reservoir. In this method anacoustic transmitter is disposed in a source well and an acousticreceiver is disposed in a producing well. A portion of the oil reservoiris disposed between the source well and the producing well. An acousticsignal is transmitted from the acoustic transmitter at frequencies of 30Hz and greater. The transmitted acoustic signal is received by theacoustic receiver and a resonant frequency of the portion of the oilreservoir is determined based on attenuation of the transmitted signal.The acoustic signal is transmitted from the acoustic transmitter at thedetermined resonant frequency to reduce a boundary layer effect betweenoil in the oil reservoir and a surface of a substrate in the oilreservoir and between the oil and a brine interface in the oilreservoir.

Another embodiment of the present invention relates to an apparatus forincreasing oil recovery from an oil reservoir. The apparatus includes anacoustic transmitter, an acoustic receiver, and a means for determininga resonant frequency. The acoustic transmitter is disposed in a sourcewell and is configured to transmit an acoustic signal at frequencies of30 Hz and greater. The acoustic receiver is disposed in a producing welland is configured to receive the transmitted acoustic signal. A portionof the oil reservoir is disposed between the source well and theproducing well. The resonant frequency of the portion of the oilreservoir is calculated by the means for determining the resonantfrequency based on attenuation of the transmitted signal. The acoustictransmitter is configured to transmit the acoustic signal at thedetermined resonant frequency to reduce a boundary layer effect betweenoil in the oil reservoir and a surface of a substrate in the oilreservoir and between the oil and a brine interface in the oilreservoir.

There has thus been outlined, rather broadly, certain embodiments of theinvention in order that the detailed description thereof herein may bebetter understood, and in order that the present contribution to the artmay be better appreciated. There are, of course, additional embodimentsof the invention that will be described below and which will form thesubject matter of the claims appended hereto.

In this respect, before explaining at least one embodiment of theinvention in detail, it is to be understood that the invention is notlimited in its application to the details of construction and to thearrangements of the components set forth in the following description orillustrated in the drawings. The invention is capable of embodiments inaddition to those described and of being practiced and carried out invarious ways. Also, it is to be understood that the phraseology andterminology employed herein, as well as the abstract, are for thepurpose of description and should not be regarded as limiting.

As such, those skilled in the art will appreciate that the conceptionupon which this disclosure is based may readily be utilized as a basisfor the designing of other structures, methods and systems for carryingout the several purposes of the present invention. It is important,therefore, that the claims be regarded as including such equivalentconstructions insofar as they do not depart from the spirit and scope ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a sonic stimulation tool acrossfrom the formation.

FIG. 2A is a graph of a raw cross well bore tomography image with commonreceiver plotted against depth and time.

FIG. 2B is a graph of a single trace plotted against amplitude and time.

FIG. 3 is a graph of an averaged Fourier Transform of all traces in asingle layer.

FIG. 4 is a graph of a typical attenuation curve shape for a singlelayer with a Q-factor of 28.

FIG. 5 is a graph of a typical attenuation curve shape for a singlelayer with a Q-factor of 4.5.

FIG. 6 is a graph of a Fourier Transform for a single trace of a guidedslow compression wave or tube wave in a thick sand layer at a centralfrequency of 317 Hz.

FIG. 7 is a graph showing the effect of acoustic stimulation on oilrecovery factor at water flood break through on the highest permeabilitylayer of an exemplary layered reservoir.

FIG. 8 is a graph showing recovery factors following core flooding,acoustic stimulation of water flood after break through, and floodingwith water and surfactant with acoustic stimulation in an exemplary Breasandstone reservoir.

FIG. 9 is a graph of a Fourier Transform for a few traces of a guidedslow compression wave or tube wave in a packet in a sand-shale layersequence having three central frequencies of 409, 503, and 578 Hz.

FIG. 10 is a cross sectional view of a heavy oil production well with asonic tool placed across the formation.

FIG. 11 is an aerial view of a 9-spot pattern under water, steam,surfactant or carbon dioxide flood.

FIG. 12 is an aerial view of a natural water drive reservoir against afault.

FIG. 13 is a graph of an average velocity ratio of first arrival shearwave versus first arrival slow compression wave of a zero offset crosswell bore tomography image of a carbon dioxide flood.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides for a method to physically determine theresonant frequency band needed to stimulate a natural water drive, waterflooded, steam flooded or CO₂ flooded oil reservoir with fluid coupledacoustic frequencies. The resonant frequency band of the slowcompression wave is a strong function of reservoir thickness, reservoirmatrix, shale layering and gas saturation. The travel time of the slowcompression wave is a function of fluid compressibility, reservoir depthand rock matrix type.

Field data indicate that the effective resonant frequency band isbounded by 30-2000 Hz and is about 80 to 120 Hz wide for mostformations. The lower bound of 50 Hz is determined by the thickness andfluid compressibility of the formation. Lower frequencies such as 10 Hzcannot be reflected off of the interface between the shale and formationbecause the wavelength greatly exceeds the thickness of the formation.The upper bound of 600 Hz is determined by the attenuation of theacoustic energy in the horizontal direction between wells. For example,a low permeability limestone with 10% porosity will transmit acousticenergy with high frequencies.

The primary method used to determine the resonant frequency band of areservoir is to conduct cross well bore tomography (cross well seismic)between source and receiver well bores. The resonant frequency band isspecific to the area between the source and receiver well bores;however, depending on formation thickness and matrix properties theresonant frequency band should range from 30-2000 Hz. The cross wellbore tomography should generate a fluid coupled compression wave in thereservoir interval to maximize Stoneley and Tube wave generation acrossthe target oil formation. Clamped casing sources will maximize the shearwave propagation of the cross well bore tomography but this methodminimizes the generation of Stoneley and Tube waves that are used tofind the resonant frequency of the formation.

A second method of determining the resonant frequency of the formationis to calculate it from monopole or dipole sonic logs that showcompression, shear and Stoneley wave arrival times. This is only anestimate and does not account for coupling between sand and shale layersor saturation, matrix and thickness changes in the reservoir.

To determine the effective coverage and the sweep efficiency in the areasurrounding the sonic stimulation source, hydrophones can be installedin offset wells to monitor the acoustic wavelet arrival for frequencyshift and velocity change. The source frequency is swept for anapproximate 100 Hz bandwidth above and below the current resonatingfrequency to verify that the sonic source well is broadcasting thecorrect resonant frequency to each offset well location. When comparingsubsequent cross well tomography surveys to original surveys in the samesource and receiver wells, frequency shifts and changes in P and S wavevelocities reveal changes in reservoir saturations between the sourceand receiver wells.

In cases where both sonic stimulation and water injection are utilized,a cationic, anionic or nonionic surfactant can be added to the injectedwater to reduce the surface tension. Core tests show that sonicstimulation by itself can lower residual saturations below 25% and thatthe addition of surfactant to water can lower residual saturation below25%, but both acting together can reduce residual oil to below 10% formost sandstone cores.

In general, the method presented in the present application is the nearresonant frequency band for the target formation is measured between atleast 2 wellbores and that frequency band is transmitted with a liquidcoupled acoustic source into the formation to reduce the residual oilsaturation by disrupting the surface tension between the oil and brinephase and disrupting the interfacial tension between the oil and thesolid pore face. The resonant frequency band is measured occasionallyand the transmission is changed to match changes in saturation and inreservoir pressure.

It is an advantage of one or more embodiments of the invention that, theresonant frequency band of a producing oil reservoir is determined orestimated. A sonic stimulation device is disposed into a well directlyacross from the producing reservoir and generate the determined resonantfrequency band in the stimulation well. The sonic stimulation causesmore oil to be mobilized and the offset producing oil wells recoversadditional oil. After the device is in operation, sound data in theoffset wells are recorded and the output frequency band is fine tuned tomatch the resonant frequency bands of the offset wells based on therecorded sound data from the stimulation device. In this manner, thestimulation process is optimized.

The main purpose of the invention is to use sonic stimulation to reducethe boundary layer effects between oil and water in the pore and betweenoil and solid surface of the pore. On a microscopic scale, during sonicstimulation, one mode is that the fluid moves in-phase with the rockmatrix and the other mode is that the fluid moves out of phase with therock matrix for maximum fluid shear against the pore surface. For highviscosity, heavy crude oils, the in-phase mode is prominent due to theviscous drag force exceeding the force required to accelerate the oildroplet. For low viscosity fluids such as water or gas, the out of phasemode is prominent. For solid tars or bitumen in the rock matrix, thereis no second fluid compression wave mode.

On a core size rock sample, sonic stimulation can reduce surface tensionbetween oil and the core matrix and reduce interfacial tension betweenoil and water with the overall effect seen as a change in wetability ofthe core (more water wet) and a reduction in residual oil. So, as thewater or gas saturation increases in the rock matrix, the shear effectfrom sonic stimulation increases and helps emulsify the oil droplets inthe displacing water phase, thus reducing residual oil saturation.

On a sand layer thickness scale, sonic stimulation can increase waterinjectivity by reducing scale damage and increasing relativepermeability by reducing residual oil in the near wellbore volume. Sonicstimulation can also increase oil productivity by reducing fines damagearound the producing well bore and mobilizing residual oil within thedrainage radius. Heat generated from electrical losses and gas bubblecompression will heat the oil in and near the well bore volume andreduce oil viscosity.

FIG. 1 is a cross-sectional view showing a sonic stimulation tool acrossfrom the formation. A Stoneley or tube wave is generated in the wellbore and is mode converted into a slow compression wave in the targetoil formation. The well is an injection, production or sonic sourcewell. For the high acoustic contrast case, shale bounds the oil sand orlimestone layer.

As shown in FIG. 1 the acoustic energy produced from the sonic sourcecan be contained in the target formation if the frequency band is chosento resonate within the target formation or internally reflect off thebounding shale layers. For fluid coupled acoustic tools, the Stoneley ortube wave generation with the target formation will improve the slowcompression wave mode conversion or coupling. The fluid coupling betweenthe sonic source and the target formation can be improved by increasingperforation density or size, hydraulically fracturing the formation, orcompleting in open hole with or without under reaming.

FIG. 2A is a graph of a raw cross well bore tomography image with commonreceiver plotted against depth and time. FIG. 2B is a graph of a singletrace plotted against amplitude and time. To acquire accuratemeasurements of frequencies and velocities in the target formation andsurrounding strata, cross well bore tomography is shot between wells inthe section of the oil field of interest. FIGS. 2A and B show a commonreceiver gather for depths ranging from 4800 ft to 5300 ft and a singletrace at 4932 ft. The three wave forms highlighted are the compressionwave, the shear wave and the guided/tube wave. The fastest acoustic wavearrival is the compression wave that has a velocity of the rock matrix.The shear wave is the next arrival along with reflections from layerboundaries. The noise in the cross well bore image surrounding thecompression and shear wave arrivals is generated from previous acousticpulses and down hole equipment from other wells in the field.

The guided, slow compression and tube waves usually arrive at 2 to 4time intervals after the shear wave arrival time. These sets of wavesare coupled to the fluid in the pore space and have velocities equal toor slower than the fluid velocity. The sonic source is swept through thelower frequencies to find the guided wave modes in the formation. Thebest guided wave mode for residual oil production is where the acousticenergy traveling in the fluid is out of phase to the acoustic energytraveling in the rock matrix.

This out of phase movement between the rock and fluid creates a shearforce on the boundary layer of fluid next to the pore surface. Withacoustic strain rates exceeding 10-6 seconds, the shear force exceedsthe surface tension or interfacial tension force between the oil andwater. With the acoustic energy canceling the surface tension force, theoil droplet can move between pores based on the pressure gradientcreated by the production wells draining the reservoir.

FIG. 3 is a graph of an averaged Fourier Transform of all traces in asingle layer. Noise (relative amplitudes below −25 db) has been removedfor clarity. Note: compression wave center frequency is 1230 Hz, shearwave center frequency is 820 Hz and the guided/tube wave centerfrequency is 385 Hz. FIG. 3 shows the Fourier transform of a singlearrival time trace averaged over all the traces for a single reservoirlayer. The compression wave (P-Wave) arrivals show an amplitude peak at1230 Hz, but there is significant acoustic energy that mode converted toa tube wave before it was recorded at the hydrophone in the receivingwell. The shear wave (S-Wave) arrivals have a peak amplitude around 820Hz. Reflected shear waves from other layers have altered their frequencyband as they traveled into this layer.

The guided, slow compression and tube waves show an amplitude peak at385 Hz. The frequencies above 600 Hz in the contour plot around the peakare probably other shear wave reflections while the frequencies below100 Hz are probably Stoneley waves generated in the well bore of thereceiving well. There is a low signal to noise ratio at these longrecord times due to multiple reflections in the reservoir and tube wavereflections in the well bore.

FIG. 4 is a graph of a typical attenuation curve shape for a singlelayer with a Q-factor of 28. Notice the negative attenuation forfrequencies between 30 and 790 Hz where higher frequency acoustic energyis transformed to lower frequency energy. FIG. 4 shows the guided waveand slow compression wave attenuation curve for a layer with a Q-factorof 28. The negative values on the attenuation curve from 30 to 790 Hzshow the layer is trapping higher frequency (1-3 kHz) acoustic energyand attenuating it into the lower frequency band. The two small dimplesat 80 Hz and 190 Hz show the first out of phase and first in-phase guidewave modes. The 80 Hz out of phase guided wave mode would be best for aproduction well because the acoustic pulse tends to pump fluid towardsthe source. The actual acoustic source should be swept from 70 to 90 Hz.The 190 Hz in-phase guided wave mode would be best for an injection wellbecause the acoustic pulse tends to pump fluid away from the source. Theactual source could be swept from 170 to 210 Hz.

FIG. 5 is a graph of a typical attenuation curve shape for a singlelayer with a Q-factor of 4.5. Notice the attenuation remains positive(acoustic energy leaking to bounding layers) and only approaches zerofor a single frequency. FIG. 5 shows the guided wave and slowcompression wave attenuation curve for a layer with a Q-factor of 4.5.This low Q-factor layer would represent a high permeability, highporosity sandstone bounded by a low-permeability siltstone in atransgressive or regressive marine strata sequence. Notice the slowcompression wave resonance at 52 Hz and the ‘leaky’ guided waveresonance at 490 Hz. Stimulation of this reservoir would be moreeffective with multiple sources due to the leakage of acoustic energyinto the bounding layers.

FIG. 6 is a graph of a Fourier Transform for a single trace of a guidedslow compression wave or tube wave in a thick sand layer at a centralfrequency of 317 Hz. For thick sandstones bounded by thick shale layers,the guided wave frequency band is very sharp due to negative attenuationconcentrating acoustic energy into the central guided wave frequency asshown in FIG. 6. The central guided wave frequency for this cross wellbore tomography example is 317 Hz. There are a number of sharp troughsin the frequency curve from 80 Hz to 390 Hz and these troughs can beresolved with layer modeling of the cross well bore tomography data. Theacoustic source should sweep between 300 to 340 Hz to stimulate the oilzone in FIG. 6.

FIG. 7 is a graph showing the effect of acoustic stimulation on oilrecovery factor at water flood break through on the highest permeabilitylayer of an exemplary layered reservoir. As shown in FIG. 7, theincrease in recovery factor of oil for high permeability layers is dueto changes in relative permeability. Thick sandstone reservoirs that areintermediate or oil wet can greatly benefit from acoustic stimulation.The increase recovery factor of oil for low permeability zones is due toincrease in absolute permeability of the zone. The vertical sweep in awater injection well would greatly benefit from acoustic stimulationbecause all the layers would have a more uniform injection profile.Another added benefit is that the scale build up in the well bore iscontinuously cleaned during sonic stimulation.

FIG. 8 is a graph showing recovery factors following core flooding,acoustic stimulation of water flood after break through, and floodingwith water and surfactant with acoustic stimulation in an exemplary Breasandstone reservoir. FIG. 8, shows a typical water flood core test foran intermediate wet rock. The ultimate recovery factor for a water floodis about 59% at 10 pore volumes injected. At 99.8% water cut, the corewas stimulated with an acoustic sweep of 100 to 120 Hz. The recoveryfactor increased to 71% at 99% water cut with 3 incremental pore volumesinjected. Then, the water surface tension was reduced with a surfactantand flooded to 99% water cut and the recovery factor was increased to81% for 6 incremental pore volumes injected. Core tests were repeatedwith 10 pore volumes injected for a water flood followed by surfactantonly and the ultimate recovery was 69% at 16 total pore volumesinjected. Intermediate wet core tests show that sonic stimulationincreases recovery by an average of 11% of original oil in place over atypical water flood with 3 incremental pore volumes injected. Oil-wetcore tests show that sonic stimulation increases recovery by an averageof 25% of original oil in place over a typical water flood with 3incremental pore volumes injected.

FIG. 9 is a graph of a Fourier Transform for a few traces of a guidedslow compression wave or tube wave in a packet in a sand-shale layersequence having three central frequencies of 409, 503, and 578 Hz. Forsequences of thin sandstone, siltstone, shale and/or limestone layers,there are multiple guided wave frequencies measured at the receivingwell as shown in FIG. 9. Notice there are only three major troughs at 65Hz, 270 Hz and 710 Hz which means a significant amount of acousticenergy is leaking from one layer to another and traveling to thereceiver well. For the example in FIG. 9, the acoustic source will needto sweep from 350 to 600 Hz to cover the entire resonate frequency band.To overcome the acoustic energy loss to bounding layers, sonicstimulation sources can be installed in closer than normal proximity toeach other.

FIG. 10 is a cross sectional view of a heavy oil production well with asonic tool placed across the formation. Worm holes created from sandproduction are used to sustain oil production rates during the primarydepletion phase of the reservoir. A Stoneley or Tube wave is generatedin the well bore to resonate in the formation and in the wormhole tohelp suspend the sand in the oil during production. Fluid filled wormholes help augment the fluid coupling between the well bore and theformation.

FIG. 10 shows a production well with a sonic source stimulating a heavyoil production zone. Large perforations and a progressive cavity pumpare used to produce the heavy oil to the surface along with theentrained sand. The sand production creates worm holes in the formationwhich in turn provide channels to drain the heavy oil to the productionwell. Stoneley waves generated in the well bore will create resonanttube waves in the worm holes. The resonant tube wave will fluidize thesand in the channel and keep the wormhole growing into the formation.The resonant tube wave will also reduce the heavy oil viscosity by afactor of 2 to 2.5 in the channel, thus reducing pressure loss aroundthe near wellbore area.

FIG. 11 is an aerial view of a 9-spot pattern under water, steam,surfactant or carbon dioxide flood. The sonic source well can be aninjector, producer, or dedicated source well. The guided wave source canstimulate multiple patterns depending on the attenuation in the oilformation.

FIG. 11 demonstrates a typical 9-spot water injection pattern using thewater injector in the middle of the pattern as a sonic source well.However, a production, an injection or a dedicated well could all serveas sonic source wells. The source spacing will depend on the guided orslow compression wave attenuation in the oil reservoir determined fromcross well bore tomography or calculation from sonic logs. The soliddots represent production wells while the open triangles represent waterinjection wells. To augment the oil recovery achieved with sonicstimulation, the surface tension of the water can be reduced by removinghardness or adding surfactant.

FIG. 12 is an aerial view of a natural water drive reservoir against afault. The sonic source well is located between the oil-water contactand the fault. The sonic source well can be a water injection well, aproduction well or a dedicated source well.

FIG. 12 is an illustration of a field where a sonic stimulation pilottest was actually performed. This is a natural water drive field withlayers dipping away from the fault and the fault itself splitting agentle anticline. The oil accumulated at the top of the gentleanticline. The field was developed with the natural water drive andproduced to 99% water cut or 1% oil cut. A sonic stimulation tool wasinstalled in a production well and the oil cut increased from 1% to 8%in a producing well near the sonic source and farther away the oil cutincreased to 4%. Wells more than 1200 ft away showed no increase in oilcut. The central frequency of the tool was 350 Hz based on cross wellbore tomography analysis of the guided waves and the tool was sweptbetween 300 and 400 Hz.

FIG. 13 is a graph of an average velocity ratio of first arrival shearwave versus first arrival slow compression wave of a zero offset crosswell bore tomography image of a carbon dioxide flood. Notice the carbondioxide has gravity segregated to the top of the oil reservoir creatinga near top-down gravity drainage displacement. As the carbon dioxidesaturation increases, the slow compression wave velocity decreases whichin turn decreases the resonant frequency of the formation.

FIG. 13 shows the effect of carbon dioxide on the slow compression wavevelocity. As carbon dioxide swells the oil and makes a second liquidphase, the compressibility of the liquid increases and the viscosity ofthe liquid decreases. Both of the effects slow the compression wavevelocity below the velocity of water in the reservoir. Sonic stimulationwill increase gravity segregation between the carbon dioxide phase andthe oil phase and enhance top-down carbon dioxide flooding in patternswith thick productive zones.

The many features and advantages of the invention are apparent from thedetailed specification, and thus, it is intended by the appended claimsto cover all such features and advantages of the invention which fallwithin the true spirit and scope of the invention. Further, sincenumerous modifications and variations will readily occur to thoseskilled in the art, it is not desired to limit the invention to theexact construction and operation illustrated and described, andaccordingly, all suitable modifications and equivalents may be resortedto, falling within the scope of the invention.

What is claimed is:
 1. A method of increasing oil recovery from an oilreservoir, the method comprising: disposing an acoustic transmitter in asource well; disposing an acoustic receiver in a producing well, whereina portion of the oil reservoir is disposed between the source well andthe producing well; transmitting an acoustic signal from the acoustictransmitter at frequencies of 30 Hz and greater; receiving thetransmitted acoustic signal; determining a resonant frequency of theportion of the oil reservoir based on attenuation of the transmittedsignal; and transmitting the acoustic signal from the acoustictransmitter at the determined resonant frequency to reduce a boundarylayer effect between oil in the oil reservoir and a surface of asubstrate in the oil reservoir and between the oil and a brine interfacein the oil reservoir.
 2. The method according to claim 1, furthercomprising: conducting cross well bore tomography between the sourcewell and the producing well to determine the resonant frequency.
 3. Themethod according to claim 1, further comprising: determining effectivecoverage and sweep efficiency of the acoustic signal by modulating theacoustic signal from about 100 Hz above the determined resonantfrequency to about 100 Hz below the determined resonant frequency andmonitoring an acoustic wavelet generated by the acoustic signal.
 4. Themethod according to claim 1, wherein the acoustic signal is transmittedat a frequency band of about 80 Hz to about 120 Hz wide.
 5. The methodaccording to claim 1, further comprising: injecting water into thesource well.
 6. The method according to claim 5, wherein the injectedwater includes a cationic surfactant.
 7. The method according to claim5, wherein the injected water includes an anionic surfactant.
 8. Themethod according to claim 5, wherein the injected water includes anonionic surfactant.
 9. The method according to claim 1, furthercomprising: re-measuring the resonant frequency of the oil reservoir andmodifying the acoustic signal in response to changes in the resonantfrequency of the oil reservoir due to saturation and reservoir pressure.10. An apparatus for increasing oil recovery from an oil reservoir, theapparatus comprising: an acoustic transmitter disposed in a source well,the acoustic transmitter being configured to transmit an acoustic signalat frequencies of 30 Hz and greater; an acoustic receiver disposed in aproducing well, wherein a portion of the oil reservoir is disposedbetween the source well and the producing well, the acoustic receiverbeing configured to receive the transmitted acoustic signal; and meansfor determining a resonant frequency of the portion of the oil reservoirbased on attenuation of the transmitted signal, wherein the acoustictransmitter being configured to transmit the acoustic signal at thedetermined resonant frequency to reduce a boundary layer effect betweenoil in the oil reservoir and a surface of a substrate in the oilreservoir and between the oil and a brine interface in the oilreservoir.
 11. The apparatus according to claim 10, further comprising:means for conducting cross well bore tomography between the source welland the producing well to determine the resonant frequency.
 12. Theapparatus according to claim 10, further comprising: means fordetermining effective coverage and sweep efficiency of the acousticsignal by modulating the acoustic signal from about 100 Hz above thedetermined resonant frequency to about 100 Hz below the determinedresonant frequency and monitoring an acoustic wavelet generated by theacoustic signal.
 13. The apparatus according to claim 10, wherein theacoustic transmitter is configured to transmit the acoustic signal at afrequency band of about 80 Hz to about 120 Hz wide.
 14. The apparatusaccording to claim 10, further comprising: means for injecting waterinto the source well.
 15. The apparatus according to claim 14, whereinthe injected water includes a cationic surfactant.
 16. The apparatusaccording to claim 14, wherein the injected water includes an anionicsurfactant.
 17. The apparatus according to claim 14, wherein theinjected water includes a nonionic surfactant.
 18. The apparatusaccording to claim 10, further comprising: means for re-measuring theresonant frequency of the oil reservoir and modifying the acousticsignal in response to changes in the resonant frequency of the oilreservoir due to saturation and reservoir pressure; and means formeasuring saturation changes in the oil reservoir due to changes in theresonant frequency.